Transferrin, the plasma protein responsible for the transport of iron in the circulation and its delivery to the erythron for the biosynthesis of hemoglobin, has two binding sites per molecule. The ligand structure of these sites will be studied by Electron Nuclear Double Resonance (ENDOR) spectroscopy and by nuclear magnetic resonance spectroscopy. Their binding affinities, and in particular the question whether they display any cooperativity in the binding of iron, will be approached by correlated equilibrium dialysis and electrophoretic methods. Finally, the interaction of transferrin with its primary receptor on the membrane of the immature erythroid cell, the physico-chemical characteristics of this receptor, and the fate of the transferrin-receptor complex during the transferrin-reticulocyte interaction, will be investigated.